Ion chromatography has become a widely used analytical technique for the determination of anionic and cationic analytes in various sample matrices since it was introduced in 1975. Even though ion chromatography today comprises a number of separation and detection modes, ion chromatography with suppressed conductivity detection remains the most widely practiced form of the technique. In suppressed conductivity detection, an eluent suppression device, or suppressor, is the critical system component used to suppress the eluent, i.e., convert the eluent into a weakly conducting form and enhance the conductance of target analytes. The original suppressors were simply columns packed with ion-exchange resins in appropriate ionic forms. Those packed-bed suppressors required frequent off-line chemical regeneration. To overcome this problem, suppressors based on ion-exchange fibers and membranes were developed. These suppressors can be continuously regenerated using either acid or base regenerant solutions.
One electrolytically-regenerated membrane suppressor is disclosed in U.S. Pat. No. 6,328,885 (the '885 patent). In this device, the suppressor includes at least one regenerant compartment and one chromatographic effluent compartment separated by an ion exchange membrane sheet. The sheet allows transmembrane passage of ions of the same charge as its exchangeable ions. Spaced electrodes are disclosed in communication with both regenerant chambers along the length of the suppressor. By applying an electrical potential across the electrodes, there is an increase in the suppression capacity of the device. U.S. Pat. No. 5,248,426 describes a suppressor of the general type described in the '885 patent in an ion chromatography system in which the effluent from the detector is recycled to the flow channel(s) in the suppressor adjacent the sample stream flow channel.
U.S. Pat. No. 6,328,885 describes methods and apparatus for increasing the current efficiency of suppressor and suppressor-like pretreatment devices similar to those described in U.S. Pat. No. 5,248,426. In one embodiment, an aqueous sample stream including analyte ions of one charge and matrix ions of opposite charge flows through a sample stream flow channel, while flowing an aqueous stream through an ion receiving flow channel separated therefrom by a first ion exchange membrane, and passing a current between the channels to reduce the concentration of the matrix ions. The sample stream flow channel has an upstream sample stream portion containing the matrix ions and an adjacent downstream portion in which the matrix ions have been suppressed. The upstream portion has an electrical resistance no greater than about 0.9 times that of the downstream portion. The ion receiving flow channel includes stationary flow-through first packing of ion exchange material. Neutral or low capacity packing may be disposed in the sample stream flow channel.
U.S. Pat. Nos. 6,325,976, 6,495,371, 6,508,985, 6,562,628, and 6,610,546 describe continuous electrolytically regenerated packed bed suppressors for ion chromatography.
U.S. Patent Application “Capillary Ion Chromatography,” Publication No. 2006/0057733, published Mar. 16, 2006 (herein, the '733 Publication), also describes several different electrolytic suppressors. In one embodiment, the suppressor includes a cation exchange capillary tubing embedded inside a bed of cation exchange resin housed in plastic column housing with flow-through ports. The inlet of the resin bed is fitted with a flow-through anode and the outlet of the resin bed is fitted with a flow-through cathode. Both electrodes are disclosed in direct contact with the resin packing. In the operation of this type of electrolytic capillary suppressor, the resin bed is continuously regenerated by hydronium ions generated through the electrolysis of water at the device anode.
The electrolytically-regenerated suppressors developed so far offer several advantages in ion chromatography. They provide continuous and simultaneous suppression of eluents, regeneration of the suppression bed, and sufficient suppression capacity for all common IC applications. They are easy to operate because the suppressed eluent or water is used to create regenerant ions electrolytically and there is no need to prepare regenerant solutions off-line. They are compatible with gradient separations. They have very low suppression zone volume, which makes it possible to achieve separations with very high chromatographic efficiency.
During the operation of ion chromatographic system, it is possible that the suppressor chromatographic effluent channel becomes exhausted if the electrical current is inadvertently turned off due to operator errors or instrument malfunctions. It is also possible that the suppressor chromatographic effluent channel become exhausted because the applied current to the suppressor is too low relative to the concentration of chromatographic eluent. The specific purpose of the suppressor stage in ion chromatography is to reduce the conductivity and noise of the analysis stream background while enhancing the conductivity of the analytes (i.e., increasing the signal/noise ratio) and maintaining chromatographic efficiency. When the suppressor chromatographic effluent channel becomes exhausted, the ion exchange sites are converted to the form of eluent cations (e.g., sodium form if a suppressor is used to suppress sodium hydroxide), analyte ions (e.g., chloride ions) exit the suppressor in the less conductive salt form (i.e., NaCl) instead of the more conductive acid form (i.e., HCl), and the conductivity of the analytes may becomes significantly smaller, which has obviously detrimental effects in analytical determination of the target analytes using ion chromatography. It is thus highly desirable that an exhausted electrolytic suppressor recovers rapidly upon application of appropriate amount of electrical current.
However, the prior-art electrolytic suppressors discussed above may not be fully regenerated rapidly upon application of electric field in the event that the chromatographic effluent channels become exhausted. In the prior-art electrolytic suppressors, the bulk of the chromatographic effluent channel is located within the electromigration pathway of regenerant ions and can be electrolytically regenerated rapidly. However, some ion exchange sites associated with outlet regions of the chromatographic effluent channels of the prior-art electrolytic suppressors may be located outside of the electromigration pathway of regenerant ions, and those ion exchange sites may not be electrolytically regenerated. In an electrolytic suppressor used for the determination of the target analyte anion (X−), the ion exchange sites in the outlet region of the suppressor chromatographic effluent channel would remain in the form of eluent cations (e.g., sodium form if it is used to suppress sodium hydroxide) even after the bulk of ion exchange sites associated with the chromatographic effluent channel are electrolytically regenerated into the hydronium form. The target analyte ions (X−) are first converted to the desired and more conductive form (H+X−) for conductivity detection as they travel thorough the bulk of the chromatographic effluent channel. As the analyte ions travel further past the ion exchange sites in the outlet region of the suppressor chromatographic effluent channel, some target analyte ions are converted into the undesired and less conductive form (Na+X−). The presence of target analyte ions in two forms (H+X− and Na+X−) in varying ratio leads the varying conductivity response of the target analyte ions and is detrimental to accurate determination of the target analyte. This behavior of the electrolytic suppressor remains until all ion exchange sites originally in the sodium form are converted into the hydronium form, which can be a slow process depending on the design of the electrolytic suppressor. Thus, the slow recovery of electrolytic suppressor can hamper its performance in ion chromatography.